Procedures for the treatment of cancer generally have been based upon the natural history of tumor spread, and thence, upon operative surgical and non-surgical options available to the physician. Surgical operative options generally have looked to the pre-, peri-, intro- and post-surgical physical identification and surgical reduction of tumors, but more recently also to the staging of the cancer's progression through the identification and evaluation of tissue to which the cancer may spread. A variety of techniques have been brought to bear in the art with the purpose of aiding the surgeon in detecting and localizing neoplastic tissue as part of these surgical procedures. (“Neoplastic tissue,” for the present purposes, often is referred to as cancerous tissue, though malignant tumors and malignant tumor cells also are found in the terminology of the art. The term “neoplastic tissue” includes all of these.) Typically, large tumors are readily located by the surgeon by visualization prior to surgery (via imaging mechanisms) and at the operating theater, and, in particular, through palpation, i.e., the feel of tumor as opposed to that of normal tissue. To increase operative success, however, it is necessary for the surgeon to locate “occult” tumors, i.e., tumors which cannot be found by preoperative imaging or the conventional surgical procedures of sight and feel. Failure to locate and remove such occult tumors generally will result in the continued growth of cancer in the patient, a condition often referred to as “recurrent” cancer.
It is generally also thought that the spread of certain types of solid tumor cancers is caused by the migration (or draining) of tumor cells from the initial tumor to nearby lymph nodes and eventually to other vital sites via the lymphatic system. Cancer surgeons and medical oncologists believe the determination of whether a patient's primary tumor has spread to the lymph nodes is a major determinant of a patient's long-term prognosis. The spread of cancer to the patient's lymph nodes is established by the examination of the nodes by pathology to determine if tumor cells are present. If tumor cells are determined to be present in the lymph nodes, the patient's stage or severity of disease is increased. Surgeons perform procedures to identify the draining node(s) through the injection of a radioactive tracing agent at the site of the primary tumor. Following injection, the tracing agent follows the drainage path of the tumor to the nearest lymph node or nodes, referred to as the “sentinel node(s).” A gamma detection device is used to detect the path of the tracing agent. Since the lymph nodes are connected, oncologists believe that if the sentinel nodes show no sign of malignancy, then the downstream nodes in the pathway are likely to be clear of disease. As such, the removal of other nearby lymph nodes would be clinically unnecessary. Therefore, the ability to rapidly locate and biopsy sentinel nodes provides vital information to the physician in determining if the cancer has spread or if it is localized to the site of the primary tumor.
Recent technologies now allow the surgeon, via a combination of both isotopically labeled drugs and hand-held radiation detection devices, to provide enhanced surgical evaluation of tumor dissemination, e.g., removal of primary tumor-associated lymph nodes. Such surgical radiation detection instrumentation is comprised generally of a hand-held probe which is in electrical communication with a control console via a flexible cable or, more recently, via wireless communication. This control console is located within the operating room facility but out of the sterile field, while the hand-held probe and forward portions of its associated cable are located within that field. The hand-held radiation detecting probe is relatively small and performs in conjunction with a semiconductor detector such as cadmium zinc tellurium or a scintillating material such as or cesium iodide. Example instrumentation may be found in U.S. Pat. No. 4,782,840, the disclosure of which is expressly incorporated herein by reference.
Radioactive sources have previously been detected directly at whatever energy levels the source of radiation is producing by using detectors comprised of semiconductor (e.g., cadmium-zinc-tellurium) or scintillating (e.g., cesium iodide) materials. An output signal is produced when an incoming photon collides with material within the detector. The higher the energy level of the primary source of radiation, the more incoming photons can pass completely through the detector without colliding with any material, thus producing no output from the detector. For this reason, high energy detectors have necessarily been made of relatively “thick” (i.e., large cross-section volume) and dense materials to assure that a sufficient number of collisions occur to provide usable detector sensitivity. This characteristic of the detector is often referred to as “stopping power” or “absorption efficiency.”
In order to more efficiently detect high-energy radiation it is usually necessary to increase the detector absorption by increasing the thickness of the detector crystal. However, thick crystals have a number of disadvantages. Firstly, the probability of defects being present in the crystal volume increases significantly with its thickness. As a result, the yield of such detector crystals is very low, making them relatively expensive to produce. In addition, the efficiency of charge collection in a detector crystal is proportional to the bias voltage applied to a detector of a given thickness. Consequently, if the thickness of the detector is increased by a given amount to increase the absorption probability, the bias voltage applied to the crystal must likewise be increased in order to maintain the same charge collection efficiency. The result is a relatively high operating voltage, which is more difficult to generate and manage, and which may present a safety hazard during use in surgery. There is a need for a cost-effective way to produce a detector crystal assembly having the absorption efficiency of a relatively thick monolithic crystal and which may be biased with a relatively low voltage.